A boom-equipped vehicle generally comprises an automotive vehicle body, a movable boom which is mounted on the vehicle body, and a work station which is mounted on the tip of the boom. The boom can be raised and lowered and extended and contracted and turned horizontally clockwise and counterclockwise on the vehicle body, and the work station can be a crane or a work platform for workmen to board. Such boom-equipped vehicles include, for example, crane trucks and aerial work platform machines. For such a boom-equipped vehicle to be used for performing a task, at first, the movable boom must be raised or lowered,extended or contracted and turned horizontally clockwise or counterclockwise to bring the work station to a desired aerial position.
While the boom is being moved, for example, being extended,the center of mass of the vehicle body shifts toward the tip of the boom, and, as a result, the moment that tends to act to tip or overturn the vehicle increases (this moment is hereinafter referred to as "tipping moment"). As the tipping moment increases, the vehicle becomes increasingly unstable and vulnerable for tipping. This is a particular problem which occurs with a boom-equipped vehicle. Therefore, a boom-equipped vehicle is generally equipped with a safety system which restricts the movement of the boom so that the tipping moment will not grow to a magnitude which actually tips the vehicle body.
Even while a boom-equipped vehicle incorporating such a safety system operates with the boom being raised and extended within a range of tolerance, there is still a danger of tipping. For example, when the boom is extended by a great amount, or when the boom is raised greatly upward though it is not extended by a large amount, the stability of the vehicle body is decreased substantially. If the vehicle in such a condition moves and encounters an upslope or a sudden difference in elevation (hereinafter referred to as "step"), then the tipping moment increases rapidly and the vehicle may overturn.
There is little problem of this kind as long as a boom-equipped vehicle travels over a flat ground. However, when the center of mass of the vehicle changes by a large amount as it encounters and moves over a step with the vehicle body being inclined, there is a danger that the vehicle may be overturned. To prevent such an accident, conventionally, there are rules. For example, a boom-equipped vehicle should not be driven over a dangerously large step (for example, a difference in elevation of 100 mm), which threatens to overturn the vehicle, or it should be driven very slowly in such a situation, notwithstanding whether the vehicle may overturn or not.
In such methods, the decision to drive the vehicle over the step or not is made by the driver with an intuition. Therefore, the driver in fear of the vehicle's overturning tends not to drive the vehicle over steps that can be safely climbed over if it is really tried. Thus, the prior-art safety system has been accompanied with this disadvantage which unnecessarily limits the utility and the workability of a boom-equipped vehicle.